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Release: March 2, 1999

UI researcher finding ways to make cochlear implants better mimic normal hearing

IOWA CITY, Iowa -- Although cochlear implants have opened up the world of sound for many individuals with profound hearing loss or deafness, the devices fall short of replicating the normal hearing process.

But that could soon change.

Using a computer model, Jay Rubinstein, M.D, Ph.D., a University of Iowa assistant professor of otolaryngology, and physiology and biophysics, has found a way to better mimic the natural spontaneous activity of the normal cochlea, which could lead to improved hearing for people who rely on cochlear implants.

Rubinstein, who figured out how to reprogram existing devices, soon will begin trials on qualified UI Hospitals and Clinics' patients already implanted with cochlear devices. An initial test of a reprogrammed implant in a North Carolina patient provided promising data, Rubinstein said. The UI Research Foundation has applied for a patent for Rubinstein's strategy.

"While much work remains to be done in humans, our laboratory studies suggest that substantial improvements in all types of sound coding may be possible, even with the limitations of currently implanted devices," Rubinstein said.

In normal hearing, sound waves excite the hair cells in the ear. These cells then convert the sound into an electrical signal that stimulates the primary auditory neurons to send the information from the inner ear's cochlea to the auditory brain stem. The hearing process is impaired in people who have lost some hair cells. Those with no hair cells at all are deaf because the hearing process cannot even start.

A cochlear implant is an electronic device that a doctor surgically places in a deaf person's cochlea. The device, which receives signals from a processor worn outside the body, acts as the hair cells, converting acoustic information into electrical signals that can be transmitted to the brain and perceived as sound.

Although the cochlear implant is supposed to mimic the normal hearing process, the electrically stimulated auditory neuron responses are substantially different from acoustically stimulated responses in the normal hearing ear. The device's electrical stimulation produces highly synchronized activity across all the neurons, meaning the response is the same for all the neurons. Acoustic stimulation produces much less across-neuron synchrony, meaning each neuron's response is independent. This independence allows for much richer, more detailed sound.

Rubinstein wanted to find a way to recreate the desynchronized, natural process of normal acoustic stimulation. By increasing the rate of the signal pulses from the external processor, Rubinstein, in his model, was able to produce more independent neuron activity. Animal studies performed at the UI support the predictions of Rubinstein's calculations. Individuals involved in the animal studies included Paul Abbas, Ph.D., professor of otolaryngology, and speech pathology and audiology; Charles Miller, Ph.D., assistant research scientist in the department of otolaryngology; and Akihiro Matsuoka , M.D., Ph.D., research assistant in the department of otolaryngology.

Earlier work performed by Blake Wilson at the Research Triangle Institute in North Carolina, a close collaborator with the UI Cochlear Implant Clinical Research Center, suggested that coding speech at high signal rates would reduce neural synchronization.

"Our work demonstrated precisely how those beneficial effects could be exploited to partially mimic the function of the normal auditory system," Rubinstein said.

If upcoming human trials are successful, a limited version of the new speech processing strategy is possible using currently implanted devices. These improved versions could be available to UI research subjects within months, Rubinstein said. U.S. Food and Drug Administration approval would be necessary prior to more widespread use. A complete implementation of the algorithm is beyond the capabilities of even the most sophisticated cochlear implants available today but will be possible using devices that the manufacturers will make available in the next few years, Rubinstein said.